During aluminum casting, an oxide film can be formed on the surface of molten aluminum supplied to a mold cavity. The formed oxide film can increase the surface tension of the molten aluminum and reduce the fluidity of the molten aluminum. The formation of an oxide film on the surface of molten aluminum thus makes it difficult to maintain the molten aluminum in good runnability.
As a casting method of maintaining molten aluminum in good runnability during aluminum casting, an “Aluminum Casting Method” disclosed in Japanese Patent Laid-Open Publication No. 2000-280063, for example, has been presented. This aluminum casting method is illustrated in FIGS. 24 and 25.
In FIG. 24, for the casting of aluminum, a nitrogen (N2) gas is first charged from a nitrogen gas cylinder 15 into a cavity 152 within a mold 151. Then, a nitrogen gas is sent into a tank 153. Magnesium powder (Mg powder) in the tank 153 is fed into a heating furnace 155 with the nitrogen gas. The magnesium powder is sublimated in the heating furnace 155. The sublimated magnesium is reacted with the nitrogen gas to form a magnesium-nitrogen compound (Mg3N2). The magnesium-nitrogen compound is injected via a pipe 156 into the cavity 152 within the mold 151. The injected magnesium-nitrogen compound is deposited on the surface of the cavity 152.
Next, molten aluminum 157 is supplied to the cavity 152. The supplied molten aluminum 157 is reacted with the magnesium-nitrogen compound to remove oxygen from oxides on the surface of the molten aluminum 157. This prevents the formation of an oxide film on the surface of the molten aluminum 157, suppressing increase in the surface tension of the molten aluminum 157. The running of the molten aluminum 157 into the cavity 152 is thus maintained in good conditions to increase the quality of aluminum casts.
Now the above-described steps of generating a magnesium-nitrogen compound and pouring molten aluminum will be described in detail.
First, the step of forming a magnesium-nitrogen compound will be described. Magnesium powder is sublimed in the heating furnace 155. The sublimed magnesium is reacted with a nitrogen gas within the heating furnace 155. Since the sublimed magnesium floats within the heating furnace 155, the nitrogen gas attaches to the entire surface of the magnesium, forming a magnesium-nitrogen compound on the entire surface.
Next, the step of pouring molten aluminum will be described with reference to FIG. 25.
A magnesium-nitrogen compound layer 159 is deposited on the surface of the cavity 152. Then the molten aluminum 157 is supplied to the cavity 152. The supply of the molten aluminum 157 to the cavity 152 brings a surface 157a of the molten aluminum 157 into contact with a surface 159a of the magnesium-nitrogen compound layer 159 for reduction to remove oxygen from an oxide 157b formed in the surface 157a of the molten aluminum 157.
Thus bringing the surface 157a of the molten aluminum 157 into contact with the surface 159a of the magnesium-nitrogen compound layer 159 removes oxygen from the oxide 157b formed in the surface 157a of the molten aluminum 157. This reveals that only the existence of the surface 159a of the magnesium-nitrogen compound layer 159 with which the surface 157a of the molten aluminum 157 is contacted is required to remove oxygen from the oxide 157b formed in the surface 157a of the molten aluminum 157.
However, as described with FIG. 24, the production of a magnesium-nitrogen compound is performed with magnesium floating within the heating furnace 155, so that the nitrogen gas attaches to the entire surface of the magnesium. The magnesium-nitrogen compound is thus produced on the entire surface of the magnesium. The magnesium-nitrogen compound deposited on the surface of the cavity 152 results in the magnesium-nitrogen compound layer 159 with film thickness t as shown in FIG. 25. This means the excessive deposition of the magnesium-nitrogen compound layer 159 on the surface of the cavity 152, resulting in time-taking formation of the magnesium-nitrogen compound layer 159, and preventing increase in productivity. Further, the excessive formation of the magnesium-nitrogen compound layer 159 results in an increase in the amount of nitrogen gas used, preventing cost reduction.
Furthermore, the above casting method adopts a method including the step of charging a nitrogen gas into the cavity. 152 with air left within the cavity 152 prior to the step of forming the magnesium-nitrogen compound layer 159 on the surface of the cavity 152. It is thus difficult to smoothly release air from the inside of the cavity 152. It therefore takes time to produce a nitrogen-gas atmosphere within the cavity 152, preventing increase in productivity.
It is thus desired to form a magnesium-nitrogen compound in a short period of time and reduce the amount of a nitrogen gas used.